1. Field of the Invention
The invention relates to a method of providing filtration of contaminants from process streams. In another aspect, this invention relates to a method for providing flow distribution of process streams in process units. In yet another aspect, this invention provides filtration or flow distribution or both while concurrently catalyzing at least one reaction to at least partially remove and/or convert certain chemical species within the process stream. In yet another aspect, this invention relates to a method and assembly for utilizing at least one cellular solid material in a component separation unit to separate one or more process streams into one or more component process streams having desired compositions.
2. Description of Related Art
Contaminants in process streams can be deleterious to processes and also to process units. Contaminants can damage process units, potentially resulting in an environmental or safety incident. Contaminants can also damage processes by decreasing efficiencies within processes, stopping production, affecting the specifications of products, or the like. Contaminants can be found in all types of process streams, such as feed streams, discharge streams, or effluent streams. Contaminants can affect various types of process units, such as reactors, extractors, distillation columns, scrubbers, tail gas treaters, incinerators, exchangers, boilers, condensers, and the like.
Process units may be configured such that process streams in the unit flows vertically downward or upward or both. Alternatively, process streams in the unit may flow radially from the center out or from the external part of the unit to the center or both.
Reactors are one type of process unit. Many reactors include discrete solid catalyst particles contained in one or more fixed beds. Catalyst beds are typically very efficient at trapping contaminants in process streams fed to the catalyst bed. Such catalyst beds, however, can quickly become clogged by these trapped contaminants. As the bed becomes clogged, pressure drop across the process unit rises resulting in eventual premature shutdown of the process unit.
Partly to mitigate this problem, catalyst bed process units as well as non-catalyst bed process units are often supplemented with conventional retention material beds that are somewhat less resistant to clogging. These conventional retention material beds are typically located at the inlet to the process unit. In the case of catalyst bed process units, the conventional retention material beds are typically inert to the reactions in the catalyst bed. These conventional retention material beds can be somewhat effective in trapping or filtering all or some contaminants such as dirt, iron oxide, iron sulfide, asphaltenes, coke fines, catalyst fines, salts, acidic impurities, sediments or other entrained foreign particulate material in the process stream entering, within or leaving the process unit. The trapping of the contaminants is to prevent undesirable material from clogging or poisoning or otherwise harming the process unit. When these conventional retention material beds are inert they are typically made of conventional ceramic materials in the form of pellets, rings, saddles or spheres and typically must be resistant to crushing, high temperatures and/or high pressures. While these conventional retention material beds can be somewhat effective in preventing the process unit from being clogged, the conventional retention material beds themselves eventually become clogged.
Conventional retention material beds may also facilitate flow distribution of the process stream in a direction perpendicular to the flow of the process stream across the process unit. Such behavior will be referred to herein as perpendicular flow distribution. As an example, in an upflow or down flow process unit, the process stream flow is in the axial direction and the perpendicular flow distribution is in the radial direction.
To increase the efficiency of conventional retention material beds, graduated layers of these materials in different sizes and shapes along with perforated discs, or screen baskets, have been used to retard the process unit from becoming clogged with contaminants such as dirt, iron oxide, iron sulfide, asphaltenes, coke fines, catalyst fines, sediments, or other entrained foreign particulate material.
Conventional retention material beds exposed to contaminants at the inlet to a process unit will eventually become clogged with contaminants. As this happens, the pressure drop across the process unit rises, resulting in the eventual shutdown of the unit. When this happens in catalyst bed process units, it is typical that part of the catalyst bed itself becomes somewhat or completely clogged with contaminants. After such shutdown of the process unit, skimming, or removal, of the clogged portion of the conventional retention material, as well as the clogged portion of the catalyst bed, is required.
In addition to clogging by contaminants in the process stream, polymerization of polymer precursors, e.g., diolefins, found in the process streams fed to catalyst bed process units may also foul, gum or plug such process units. In particular, two mechanisms of polymerization, free radical polymerization and condensation-type polymerization, may cause catalyst bed fouling, gumming or plugging. The addition of antioxidants to control free radical polymerization has been found useful where the process stream has encountered oxygen. Condensation polymerization of diolefins typically occurs after an organic-based feed is heated. Therefore, filtering prior to the process stream entering the catalyst bed process unit may not be helpful to remove these foulants as the polymerization reactions generally take place in the unit.
It is highly desirable to have retention materials that do not just clog with contaminants but efficiently and effectively filter contaminants from the process stream. Efficiency relates to the percent of contaminants removed by such materials from the process stream, as well as, to the range of sizes of contaminants that can be removed by such materials. Effectiveness relates to the extent that such materials do not impede the flow of the decontaminated process stream through the retention materials. Such materials would desirably remove virtually all contaminants within a broad range of sizes from the process stream, while not causing an unacceptable pressure drop increase across the process unit. It is also highly desirable to have retention materials that promote perpendicular flow distribution. The method of the present invention for filtration and flow distribution for process streams, when compared with previously proposed prior art methods, has the advantages of providing highly efficient and highly effective filtering of contaminants; increasing the life and activity of catalysts in catalyst bed process units; decreasing catalyst losses; enhancing product selectivities, increasing throughput/productivity, allowing for the optimization of process unit configuration; improving the perpendicular flow distribution of process streams into and within process units and eliminating the need to take process units off-line when conventional retention material beds have clogged to the point that pressure drop across units have risen to unacceptable levels. These benefits will result in both capital and operating cost savings, reduced downtimes, increased process unit performance and extended process unit operating time.
Weaknesses of conventional retention material beds are that they are neither particularly efficient nor particularly effective as filters. Conventional retention material beds are typically efficient at removing some contaminants from the process stream for a limited period of time. The contaminants so trapped are typically those about 50 microns and larger. The effectiveness of conventional retention material beds suffers due to eventual clogging, which prevents flow of the decontaminated process stream through the conventional retention material beds and leads to unacceptable increase in process unit pressure drop. Furthermore, conventional retention material beds appear to trap contaminants within about the top six to twelve inches of depth. Deeper beds of conventional retention materials do not increase the trapping capacity of these materials. Therefore, the art has sought filtration methods that remove particulate contaminants smaller than 50 microns, that filter particulate contaminants while allowing the free flow of decontaminated process streams with no significant rise in process unit pressure drop and that have a filtering capacity that increases with bed depth, regardless of bed depth.
Disadvantages associated with current perpendicular flow distribution designs and methods in process units may result in poor distribution within the process unit. Clogging or other fouling such as that caused by particulate contaminants or the products of undesired polymerization reactions may also cause maldistribution. The maldistribution may result in channeling and corresponding bypassing of portions of the process unit, reduction in the efficiency of contaminant removal and reduction in unit efficiency. Usually, a maldistribution problem is also evidenced by so-called temperature hot-spots. Such hot-spots can, for example, lead to increased coking and reduced activity in catalyst bed process units. Besides maldistribution problems and coking, the increase in pressure drop may cause catalyst breakdown as a result of attrition. Therefore, the art has sought a perpendicular flow distribution method that may distribute the process stream more uniformly within the process unit, provide efficient filtering of contaminants, reduce the occurrence of hot-spots, minimize catalyst attrition, and reduce fouling caused by undesired polymerization reactions.
U.S. Pat. Nos. 6,258,900 and 6,291,603, both of which are incorporated by reference in their entireties, describe reticulated ceramic materials that are used to filter and distribute organic feed streams in a chemical reactor. A need exists for filtering and flow distribution capabilities for other types of process streams besides organic-based streams and for other types of process units besides chemical reactors.
It is desirable for the filtering and flow distribution methods for all process streams and all process units to increase the filtering efficiency and effectiveness of materials utilized to remove contaminants from process streams, to improve perpendicular flow distribution within process units, to have unit run length determined by factors other than pressure drop increase, to minimize pressure drops across process equipment, and to maximize process safety and minimize environmental concerns arising from catalyst bed channeling and flow misdistribution, temperature hot-spots and process unit shutdowns and start-ups.
Component separation units are a specific type of process unit that have traditionally been used in laboratories, pilot plants and industrial facilities to separate process streams into component process streams having desired compositions. With regard to any component separation unit, a “process stream” can refer to a feed stream, “component process streams” can refer to product streams from the unit and “phases” can refer to individual liquid or vapor phases within the unit. During component separation, a phase moving in one direction and a phase moving in the opposite direction are contacted with one another within the component separation unit to effectuate mass transfer at the interface between the phases. Component separation is accomplished as a result of this mass transfer. As a result of the mass transfer, one or more process streams are separated to form one or more component process streams each having desired compositions. Typically, a plurality of trays and/or packing elements are positioned within the unit to facilitate contact between the phases and mass transfer between the phases. The trays are typically stacked horizontally with respect to one another, while the packing elements are randomly loaded or formed into a structured shape. Randomly loaded packing elements generally do not have any specified orientation relative to one another, while structured elements have a specific overall shape and relative orientation.
Examples of component separation units include, for example, distillation units, chromatographic units, absorbers, extractors and combinations thereof. Distillation units achieve component separation based on the differences in boiling points of the species present in the process streams fed to the unit. Distillation units include, for example, columns, fractionators, splitters, semi-continuous units, continuous units, flash units, batch distillation units, strippers, rectifiers, extractive distillation units, azeotropic distillation units, and vacuum distillation units. Absorbers and extractors are contacting units in which vapor and liquid phases are contacted and desired component separation is achieved based on the affinity of components in one phase to the components in the other phase. For example, a process stream containing components A and B may enter such a unit at one position while another process stream containing C may enter the unit at another position. One of these streams is typically liquid while the other can be liquid or vapor. Now assume component B has a much greater affinity for component C than for component A. Intimate contacting of the two streams in a properly designed and operated contacting unit will result in creating one product stream containing component A with a essentially no component B and a second product stream containing component C and essentially all of component B. Commercial use of such a unit might be driven by the difficulty of directly separating B from A versus of separating B from C. In this example the first product stream would be termed the desorbant and the second product stream would be termed the absorbant. Specific examples of absorber units include continuous absorbers, temperature swing absorbers, pressure swing absorbers, purge/concentration swing absorbers and parametric pumping. Extractors are contacting units in which immiscible liquid phases are contacted and component separation is achieved using a mass separating agent. In the example above, the component C in the second process stream would be the mass separating agent. An example of an extractor unit is an aromatics extraction unit wherein a hydrocarbon stream containing aromatic species and non-aromatic species are contacted with a mass separating agent such as sulfolane or morpholine and efficient contacting of these two immiscible liquids results in extraction of the aromatic species from the hydrocarbon steam into the stream containing the mass separating agent. Component separation units can also include a zone of catalytic materials to facilitate desired chemical reactions in the component separation unit. Examples of such include reactive distillation units and extractive distillation units. Examples of conventional unit internals used to achieve or enhance separation in component separation units include, for example, trays, randomly packed rings or saddles, structured packing having meshes, monoliths, gauzes and the like, collectors, distributors, downcomers, wall wipers, support grids and hold down plates.
Inside a component separation unit, there is repeated intimate contact between the rising phase and the falling phase. This contact is facilitated by the trays and/or packing materials. Each section of trays or depth of packing material may approximately represent a number of “theoretical stages” of separation. The component separation unit internals are designed and positioned within the component separation unit to produce the appropriate number of “theoretical stages” that will achieve the desired separation.
In distillation units the repeated contact between the phases ultimately results in a vapor phase consisting of higher volatility, lower boiling point species and a liquid phase consisting of lower volatility, higher boiling point species. This mass transfer between phases is driven by the differences between the boiling points of the species in the phases. Species with lower boiling points rise and components with higher boiling points fall. Upon creation of the one or more phases of desired composition, a portion of the vapor phase is typically recovered as an upper component process stream, and the remaining portion is condensed and passed as a reflux phase back into the distillation unit for further mass transfer. Likewise, a portion of the liquid phase is recovered as a lower component process stream, and the remaining portion is reboiled (i.e., vaporized) and returned to the distillation unit for further mass transfer. In addition, one or more component process streams can be recovered from the distillation unit at any location between the top and bottom of the distillation unit.
In component separation units, it is highly desirable to achieve efficient and cost effective separation within the unit. It is also highly desirable to achieve low pressure drop within the unit and a low HETP (height equivalent to a theoretical plate (or stage)) number for the unit. The degree of separation achieved by the unit may be affected by, among other factors, the amount of contact between the phases, the number of trays used, the amount and type of packing material used, the temperature and pressure at which the unit is operated, and the differences between the boiling points or other relevant separation characteristics of the species contained within the phases. Separation may also be affected by, for example, the design of the trays, the use of distributors in the unit to promote uniform distribution of phases across the cross-sectional area of the unit, and the design of the packing materials.
Prior art packing materials within component separation units have been either randomly loaded or structured. Randomly loaded or “loose” packing, although less costly than structured packing material, has been shown to have high pressure drop or low mass transfer characteristics, and suffer from poor phase distribution which results in poor separation efficiency in the unit. Also, prior art units that have utilized trays or “loose” packing materials have proven to be prone to corrosion and fouling and have provided inefficient separation. As a result, prior art loose packing technology gave way to the development of highly engineered structured packing technology. Structured packing materials can provide improved or separation efficiency; however, the manufacturing of structured packing material requires sophisticated machineries, engineering expertise and fabrication skills to design larger units to perfection. Further, these materials are generally more expensive to fabricate and require more unit down time for installation than random packing. Even though they are more costly, structured packing materials are often used in place of random packing because they provide higher production rates than existing units due to better pressure drop and mass transfer characteristics. The use of structured packing materials, however, has generally been limited to processes that are not subject to fouling or corrosion. Structured packing is more expensive and difficult to install, and so its use in processes where fouling or corrosion would necessitate more frequent replacement is economically unattractive.
Accordingly, prior to the development of the present invention, there has been no method and apparatus for separating process streams into component process streams having desired compositions in a component separation unit which provides the desirable characteristics and/or levels of: efficient separation at a low HETP value; relatively low pressure drop; resistance to fouling and/or corrosion; low fabrication and installation costs; ease of replacement; and improved overall performance and production. Therefore, the art has sought a method and apparatus for improving the separation of process streams into desired component process streams via distillation, absorption and/or extraction which: does not cause relatively large pressure drops; displays more efficient separation at a low HETP number; requires less complex and expensive design, fabrication, installation, operation and maintenance, resists fouling and corrosion, can be easily replaced and exhibits overall improved performance and production.